This invention relates to an electrodeless spark discharge acoustic source transducer for borehole operations.
Spark discharge devices have been employed in the prior art to generate acoustic pulse signals in liquid media such as seawater for the purpose of conducting bathymetric surveys of the ocean and subbottom profiles of marine sediments. In these applications, the metallic spark discharge electrodes are immersed directly in the salt water electrolyte with the result that the repetitive plasma discharge erodes the electrodes to change their geometric shape and spacing. The spark discharge occurring in such open water environments is also subject to possible spatial variations in the electrolytic parameters of the liquid medium. Electrode geometry, spacing, and electrolyte parameters affect the electro-acoustic energy conversion process associated with the electric spark discharge, including the accuracy and stability of the timing between the spark-initiating trigger and the resulting acoustic pressure pulse.
The use of open-electrode spark discharge devices in waterfilled boreholes has been reported. However, since the fluid present in such boreholes is usually freshwater, which is only slightly conductive, salt must be added to the boreholes in order to obtain efficient electrical discharge operation. Even when such conditioning of the borehole fluid is performed, the spark discharge process is still subject to the same variables mentioned in connection with the spark discharges operating in open seawater. Moreover, while acoustic pulse signals generated by spark discharge techniques can potentially provide a useful and efficient means of probing the geologic materials from boreholes, many drill holes are unable to hold water because of geologic fractures or because of their horizontal orientation.
U.S. Pat. No. 3,428,940 to Huckabay discloses a sonic transmitter having a housing containing a chamber separated from the water or other medium through which sound is to be transmitted. The housing has an elastic outer wall positioned at one end of the transmitter. The chamber has a body of liquid metal contained in the chamber in contact with a pair of spaced, solid electrodes to form a conductor between the solid electrodes. The transmitter also includes a source of electrical energy connected to the solid electrodes and sized to impose a potential across the electrodes sufficient to momentarily change the state of the liquid metal to a gas, such that the elastic wall of the housing is expanded outwardly after the liquid metal vaporizes and a sonic wave is generated in the surrounding water or other medium. Because the outer wall is positioned at one end of the transmitter, the transmitter provides a generally unidirectional acoustic wave.